U.S. patent number 9,651,199 [Application Number 15/407,035] was granted by the patent office on 2017-05-16 for hydrogen supply method and system.
This patent grant is currently assigned to PRAXAIR TECHNOLOGY, INC.. The grantee listed for this patent is Solomon Dadebo, Rommel M. Oates. Invention is credited to Solomon Dadebo, Rommel M. Oates.
United States Patent |
9,651,199 |
Oates , et al. |
May 16, 2017 |
Hydrogen supply method and system
Abstract
A method and system for supplying additional hydrogen from a
reservoir of stored hydrogen in a salt cavern to a hydrogen
pipeline to assist in meeting customer demand for hydrogen is
provided. Contaminants introduced while the stored hydrogen stream
is in the salt cavern may cause the crude hydrogen stream to not
have the required product purity specification. The stored hydrogen
is removed from the salt cavern as a crude hydrogen stream and
thereafter diluted with higher purity hydrogen formed from the
pipeline to form a hydrogen product stream at or below the product
purity specification. The hydrogen product can be formed without
removal of any of the contaminants in the crude stream, thereby
creating a more cost effective and simplified supply process
compared to conventional processes employing a salt cavern for
hydrogen supply.
Inventors: |
Oates; Rommel M. (Spring,
TX), Dadebo; Solomon (Spring, TX) |
Applicant: |
Name |
City |
State |
Country |
Type |
Oates; Rommel M.
Dadebo; Solomon |
Spring
Spring |
TX
TX |
US
US |
|
|
Assignee: |
PRAXAIR TECHNOLOGY, INC.
(Danbury, CT)
|
Family
ID: |
47755013 |
Appl.
No.: |
15/407,035 |
Filed: |
January 16, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
14574945 |
Feb 18, 2014 |
9574715 |
|
|
|
13398482 |
Feb 10, 2015 |
8950419 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F17D
1/04 (20130101); F17D 1/17 (20130101); F17C
1/007 (20130101); B65G 5/00 (20130101); F17D
1/02 (20130101); F17D 3/00 (20130101); F17D
1/00 (20130101); F17D 3/16 (20130101); F17C
13/02 (20130101); Y02E 60/321 (20130101); F17C
2227/00 (20130101); Y02E 60/322 (20130101); Y10T
137/0318 (20150401); F17C 2223/0123 (20130101); Y10T
137/2509 (20150401); Y02P 90/45 (20151101); F17C
2221/012 (20130101); Y10T 137/402 (20150401); Y02E
60/32 (20130101); Y02E 60/34 (20130101); Y10T
137/87676 (20150401); Y10T 137/0329 (20150401); Y10T
137/7025 (20150401) |
Current International
Class: |
F17D
3/00 (20060101); F17D 1/02 (20060101); B65G
5/00 (20060101); F17C 1/00 (20060101) |
Field of
Search: |
;137/3,4,88,93,236.1
;405/53,59 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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58 191399 |
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Nov 1983 |
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JP |
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WO 02-097321 |
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Dec 2002 |
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WO |
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Primary Examiner: Reid; Michael R
Attorney, Agent or Firm: Dalal; Nilay S.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of and claims the benefit of
priority from U.S. application Ser. No. 14/574,945, filed Dec. 18,
2014, which, in turn, is a continuation of and claims the benefit
of priority from U.S. application Ser. No. 13/398,482, filed Feb.
16, 2012, now U.S. Pat. No. 8,950,419, each which is incorporated
by reference in its entirety.
Claims
The invention claimed is:
1. A method for supplying hydrogen from a salt cavern to assist in
meeting end-user demand for hydrogen at a purity of greater than
99% that is supplied by a pipeline, the method comprising: removing
a crude hydrogen stream from the salt cavern having a first flow
rate and containing at least one contaminant imparted by the salt
cavern such that the at least one contaminant in the crude hydrogen
stream does not meet a pre-established pipeline product purity
specification; mixing the crude hydrogen stream with a hydrogen
dilution stream having a second flow rate to form a hydrogen
product stream, the hydrogen dilution stream formed from hydrogen
in the pipeline and containing each of the at least one
contaminants lower than the pre-established pipeline product purity
specification; controlling the first flow rate of the crude
hydrogen stream or the second flow rate of the hydrogen dilution
stream such that the hydrogen product stream contains hydrogen at
the purity of greater than 99% with the at least one contaminant
found in the crude hydrogen stream at or below the pre-established
pipeline product purity specification; and supplying the hydrogen
product stream to the end-user.
2. The method of claim 1, wherein the hydrogen in the hydrogen
product stream has a purity greater than 99.9%.
3. The method of claim 1, wherein the second flow rate of the
hydrogen dilution stream is controlled by: sampling concentration
levels of the at least one contaminants in the crude hydrogen
stream to generate a first measurement of the concentration levels;
sampling concentration levels of the at least one contaminants in
the hydrogen dilution stream to generate a second measurement of
the concentration levels; utilizing the first measurement of the
concentration levels and the second measurement of the
concentration levels along with the first flow rate of the crude
hydrogen stream to determine a calculated value of concentration
levels of the at least one contaminants in the hydrogen product
stream by performing material balances for each of the at least one
contaminants in the crude hydrogen stream such that the calculated
level of the contaminants in the hydrogen product stream will be at
or below the pre-established product purity specification.
4. The method of claim 3, wherein integrity checks are performed to
assess reliability of the first measurement of the concentration
levels and the second measurement of the concentration levels.
5. The method of claim 3, further comprising: sampling
concentration levels of the contaminants in the hydrogen product
stream to generate a third measurement of the concentration levels;
correcting the third measurement with a trailing bias value to
generate a reconciled value; determining deviations between the
reconciled value and the calculated value of concentration levels
of contaminants in the hydrogen product stream to generate model
errors; updating the trailing bias with the model errors for
subsequent use of the trailing bias; and exporting the hydrogen
product stream when the model errors are less than acceptable
limits.
6. The method of claim 1, wherein stored hydrogen in the cavern is
at a pressure that is above the pipeline pressure, such that at
least a portion of the stored hydrogen is removed from the salt
cavern as the crude hydrogen stream under impetus of the cavern
pressure being higher than that of the pipeline pressure.
7. The method of claim 1, wherein none of the contaminants
contained in the crude hydrogen stream are removed prior to mixing
by the hydrogen dilution stream.
8. The method of claim 1, wherein the at least one contaminant
contained in the crude hydrogen stream is partially removed prior
to mixing by the hydrogen dilution stream.
9. The method of claim 8, wherein the at least one contaminant
comprises water vapor.
10. A method for supplying additional hydrogen to a hydrogen
pipeline from a salt cavern to assist in meeting end-use demand,
the method comprising: removing stored hydrogen from the salt
cavern at a first flow rate as a crude hydrogen stream containing
at least one contaminant comprising carbon dioxide or water vapor
contaminants at a level higher than that allowed by a product
purity specification for the hydrogen in the hydrogen pipeline; and
mixing the crude hydrogen stream at the first flow rate with a
higher purity hydrogen from the hydrogen pipeline having a second
flow rate under flow rate conditions to form a hydrogen product
stream comprising hydrogen at a purity of greater than 99% with at
least the carbon dioxide or the water vapor contaminants at
concentrations at or below the product purity specification.
11. The method of claim 10, wherein a flow network is positioned
between the hydrogen pipeline and the salt cavern to remove the
higher purity hydrogen from the hydrogen pipeline to store in the
salt cavern as the stored hydrogen, and to supply at least a
portion of the stored hydrogen from the salt cavern to the hydrogen
pipeline.
12. The method of claim 10, wherein at least part of the hydrogen
product stream is supplied to an end-user.
13. The method of claim 10, wherein the first flow rate of the
crude hydrogen stream is regulated to a target flow rate set by a
control system.
14. The method of claim 13, wherein the control system sets the
target flow rate on the basis of impurity concentration
measurements taken within the crude hydrogen stream.
15. The method of claim 14, wherein the impurity concentration
measurements are taken with an analyzer bank containing sensing
elements.
16. The method of claim 15, wherein for each contaminant having an
impurity level that exceeds the product purity specification, the
control system performs overall material balance and contaminant
material balances to calculate the second flow rate of the higher
purity hydrogen required for reducing at least the carbon dioxide
or the water vapor contaminants at or below the product purity
specification.
17. The method of claim 10, wherein the product purity
specification requires a balance of the contaminants to comprise at
least light hydrocarbons, carbon monoxide or hydrogen sulfide at or
below the product purity specification.
18. The method of claim 10, further comprising using a control
system that controls the first flow rate of the crude hydrogen
stream or controls the second flow rate of the higher purity
hydrogen.
19. The method of claim 18, further comprising using the control
system to regulate the second flow rate of the higher purity
hydrogen to a target flow rate.
20. The method of claim 10, wherein none of the contaminants
contained in the crude hydrogen stream are removed prior to mixing
by the higher purity hydrogen.
21. The method of claim 10, wherein at least a portion of the water
vapor contaminant contained in the crude hydrogen stream is removed
prior to mixing with the higher purity hydrogen.
22. The method of claim 10, wherein the at least one contaminant in
the crude hydrogen stream comprises water vapor and carbon
dioxide.
23. The method of claim 10, wherein the higher purity hydrogen
comprises hydrogen at a purity of greater than 99.9% and further
comprises at least the carbon dioxide and the water vapor
contaminants at a concentration that is no greater than 80% of the
carbon dioxide and water vapor contaminant levels in the product
purity specification.
Description
FIELD OF THE INVENTION
The present invention relates to a hydrogen supply method and
system for supplying additional hydrogen to a hydrogen pipeline
from a salt cavern to assist in meeting customer demand for
hydrogen. More particularly, the present invention relates to such
a method and system in which the stored hydrogen is removed from
the salt cavern as a crude hydrogen stream that is thereafter
diluted with higher purity hydrogen from the pipeline to form a
hydrogen product stream at or below a product purity
specification.
BACKGROUND OF THE INVENTION
Hydrogen is supplied to customers connected to a hydrogen pipeline
system. Typically, the hydrogen is manufactured by steam methane
reforming in which a hydrocarbon and steam are reacted at high
temperature in order to produce a synthesis gas containing hydrogen
and carbon monoxide. Hydrogen is separated from the synthesis gas
to produce a hydrogen product stream that is introduced into the
pipeline system for distribution to customers that are connected to
the pipeline system. Alternatively, hydrogen produced from the
partial oxidation of a hydrocarbon can be recovered from a hydrogen
rich stream. Typically, hydrogen is supplied to customers under
agreements that require availability and on stream times for the
steam methane reformer or hydrogen recovery plant. When a steam
methane reformer is taken off-line for unplanned or extended
maintenance, the result could be a violation of such agreements.
Additionally, there are instances in which customer demand can
exceed hydrogen production capacity of existing plants. Having a
storage facility to supply back-up hydrogen to the pipeline supply
is therefore desirable in connection with hydrogen pipeline
operations. Considering that hydrogen production plants on average
have production capacities that are roughly 50 million standard
cubic feet per day, a storage facility for hydrogen that would
allow a plant to be taken off-line, to be effective, would need to
have storage capacity in the order of 1 billion standard cubic feet
or greater.
The large storage capacity can be met by means of salt caverns to
store the hydrogen underground. Hydrogen as well as other gases
have been stored in salt caverns. Salt caverns are large
underground voids that are formed by adding fresh water to the
underground salt, thus creating brine, which is often referred to
as solution mining. Caverns are common in the gulf states of the
United States where demand for hydrogen is particularly high. Such
hydrogen storage has taken place where there are no purity
requirements or less stringent (<96% pure) requirements placed
upon the hydrogen product. In such case, the stored hydrogen from
the salt cavern is simply removed from the salt cavern without
further processing. Hydrogen storage has also occurred where there
are more stringent purity requirements for the hydrogen within the
pipeline, and, therefore, for the hydrogen that has been previously
stored in the salt cavern that is to be introduced into the
pipeline. In order to comply with the more stringent purity
requirements, the stored hydrogen that is removed from the pipeline
needs to be further processed to remove contaminants that have been
imparted into the stored hydrogen by virtue of its storage within
the salt cavern. For instance, U.S. Pat. No. 7,078,011 discloses a
temperature swing adsorption unit for removing carbon dioxide and
water from a hydrogen stream that has been stored in a salt cavern
to produce a hydrogen product stream having the impurity level of
the carbon dioxide and water vapor at or below the product purity
specification. Such a purified hydrogen product stream can then be
reintroduced into the pipeline. However, the implementation of
purification equipment can substantially increase the cost and
complexity of operating a hydrogen storage cavern.
As will be discussed, among other advantages of the present
invention, a method and system for processing hydrogen that has
been stored in a salt cavern is disclosed in which the hydrogen to
be reintroduced into the pipeline will assist in meeting customer
demand in a manner that is more cost effective than methods
disclosed in the prior art.
SUMMARY OF THE INVENTION
The present invention requires hydrogen from the hydrogen pipeline
to mix with the crude hydrogen stream. The crude hydrogen stream
that is withdrawn from salt cavern is diluted to an acceptable
product purity so as to enable assisting with customer demand. As
will be explained, the crude hydrogen stream does not solely meet
the customer demand, but, rather, relies on dilution with a
predetermined portion of hydrogen from the hydrogen pipeline.
The present invention utilizes a method for processing a crude
hydrogen stream withdrawn from stored hydrogen within a salt
cavern. The crude hydrogen stream can be withdrawn at a flow rate
to satisfy the hydrogen demand in a hydrogen pipeline. The crude
stream contains at least one contaminant below a minimum level
allowed by a product purity specification. A predetermined portion
of a hydrogen product stream from the hydrogen pipeline mixes with
the crude hydrogen stream. The mixture of the hydrogen product
stream and the crude hydrogen stream has an acceptable product
purity level suitable for downstream usage to an end-user. In this
way, the present invention offers a more cost effective and
simplified approach for operating a salt cavern containing stored
hydrogen with at least one contaminant imparted by the salt
cavern.
In a first aspect, a method is provided for supplying hydrogen from
a salt cavern to assist in meeting end-user demand for hydrogen at
a purity of greater than 99% that is supplied by a pipeline. The
method comprises removing a crude hydrogen stream from the salt
cavern having a first flow rate and containing at least one
contaminant imparted by the salt cavern such that the at least one
contaminant in the crude hydrogen stream does not meet a
pre-established pipeline product purity specification. The crude
hydrogen stream is mixed with a hydrogen dilution stream having a
second flow rate to form a hydrogen product stream. The hydrogen
dilution stream is formed from hydrogen in the pipeline and
contains each of the at least one contaminants lower than the
pre-established pipeline product purity specification. The first
flow rate of the crude hydrogen stream or the second flow rate of
the hydrogen dilution stream is controlled such that the hydrogen
product stream contains hydrogen at the purity of greater than 99%
with the at least one contaminant found in the crude hydrogen
stream at or below the pre-established pipeline product purity
specification. The hydrogen product stream is supplied to the
end-user.
In a second aspect, a method is provided for supplying additional
hydrogen from a reservoir of stored hydrogen in a salt cavern
supplied at a purity of greater than 99% to an end-user through a
hydrogen pipeline. The method comprises removing a first stream
from the salt cavern at a first flow rate to discharge crude
hydrogen from the salt cavern. The first stream comprises hydrogen
and at least one contaminant imparted by the salt cavern such that
the crude hydrogen contains the at least one contaminant at a level
higher than that allowed by a product purity specification. The
first stream is mixed at the first flow rate with a second stream
comprising the at least one contaminant at or within the product
purity specification at a second flow rate under flow rate
conditions that form a third stream having lower levels of the at
least one contaminant than contained in the first stream.
In a third aspect, a method is provided for supplying additional
hydrogen to a hydrogen pipeline from a salt cavern to assist in
meeting end-use demand. The method comprises removing stored
hydrogen from the salt cavern at a first flow rate as a crude
hydrogen stream containing at least one contaminant comprising at
least carbon dioxide or water vapor contaminants at a level higher
than that allowed by a product purity specification for the
hydrogen in the hydrogen pipeline. The crude hydrogen stream is
mixed at the first flow rate with a higher purity hydrogen from the
hydrogen pipeline having a second flow rate under flow rate
conditions to form a hydrogen product stream comprising hydrogen at
a purity of greater than 99% with at least the carbon dioxide or
the water vapor contaminants at concentrations at or below the
product purity specification.
BRIEF DESCRIPTION OF THE DRAWINGS
The objectives and advantages of the invention will be better
understood from the following detailed description of the preferred
embodiments thereof in connection with the accompanying figures
wherein like numbers denote same features throughout and
wherein:
FIG. 1 shows a fragmentary schematic of a mixing process for
storing and supplying hydrogen to a hydrogen pipeline incorporating
the principles of the present invention; and
FIG. 2 shows a control logic incorporated into a control system
that can be utilized for carrying out a method in accordance with
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
As used herein and in the claims, all concentrations are expressed
as volumetric or mole percentages. With reference to FIG. 1, a
hydrogen storage and processing facility 1 is illustrated that is
designed to remove hydrogen from a hydrogen pipeline 2 during
periods of low customer demand and store such hydrogen within a
salt cavern 3 as stored hydrogen 4. During periods at which demand
for the hydrogen exceeds the capabilities of the hydrogen pipeline
2 to supply hydrogen to customers, stored hydrogen 4 is removed
from the salt cavern 3 and used to assist in meeting the customer
demand that cannot otherwise be supplied by production facilities
feeding the hydrogen pipeline 2. In a manner that will be
discussed, an outlet flow network 5 is provided to remove stored
hydrogen 4 from the salt cavern 3 and then process and reintroduce
the hydrogen so obtained from the salt cavern 3 back into the
hydrogen pipeline 2 to provide a supply of additional hydrogen to
assist in meeting customer demand. The ability to assist meeting
customer demand may be advantageous when one or more hydrogen
generation facilities, such as for instance a steam methane
reformer, is taken off-line or is required to divert hydrogen
supply from a particular customer to another end-use application,
such as for example, an oil patch production facility. It is to be
noted that even when customer demand for hydrogen is not high or
where demand is high, the stored hydrogen 4, after processing, can
be sold in merchant applications and, therefore, not all of the
stored hydrogen 4 removed from the salt cavern 3 is necessarily
reintroduced into the hydrogen pipeline 2.
As known in the art, salt cavern 3 is formed by solution mining in
which water is injected through a pipe known as a brine string 10.
The water dissolves the salt, and the resulting brine during the
mining operation is returned through the annular space (not shown)
formed in the final well casing 12 or other conduit between the
inner wall of the final well casing 12 and the outer wall of the
brine string 10. After the solution mining operation is complete,
the residual brine in the salt cavern 3 can be removed through the
brine string 10 by pressure displacement resulting from injection
of hydrogen through the final casing 12 or other conduit. Once the
brine level reaches the bottom of the brine string 10, a top
section 14 of the brine string 10 is sealed off by a valve 16 and a
residual brine layer 20, also known as a brine sump, remains in the
bottom of the salt cavern 3.
As is also known in the art, contaminants will invariably be
introduced into the stored hydrogen 4 thereby causing the stored
hydrogen 4 in the salt cavern 3 to no longer meet the
pre-established pipeline product purity specifications. The
contamination can be in the form of a mixture of several light
hydrocarbons, carbon dioxide, hydrogen sulfide and water vapor that
is introduced into the stored hydrogen 4 from the walls of the salt
cavern 3 and the residual brine layer 20. By way of an example,
although the hydrogen introduced into the salt cavern 3 can have a
hydrogen purity of about 99.99%, the purity of the stored hydrogen
4, will fall to less than about 99.99% due to the introduction of
the following contaminants: carbon monoxide and carbon dioxide
greater than about 1 ppmv; nitrogen and argon less than about 100
ppmv; total hydrocarbon contamination ("THC") of greater than about
1 ppmv; oxygen contamination of less than about 1 ppmv; water vapor
greater than about 1 ppmv; and hydrogen sulfide of about 1
ppmv.
The product purity specification for the hydrogen that is to be
drawn from the hydrogen pipeline 2 is typically established by the
end-user needs and will generally be greater than about 95%
hydrogen purity. Product purity specifications can range between
about 97% and about 98%, and, in some instances, can be greater
than about 99%. The product purity specification for the hydrogen
pipeline is in line with the foregoing end-user requirements and,
therefore, the hydrogen purity for such purposes can be greater
than about 99.99%, with the nitrogen and argon specification less
than about 100 ppmv and the balance of the contaminants less than
about 1 ppmv.
When customer demand for the hydrogen, supplied by hydrogen
pipeline 2 is low or for any reason, production exceeds demand, a
hydrogen stream 30 can be removed from the hydrogen pipeline 2 and
introduced into the salt cavern 3. In this regard, a bypass valve
56 is set in a closed position, a valve 35 is set in an open
position and a control valve 44, to be discussed, is set in a
closed position to allow hydrogen stream 30 to be compressed in a
hydrogen compressor 32 to produce a compressed hydrogen stream 34.
Hydrogen compressor 32 is typically a compressor having a piston
that reciprocates to compress the hydrogen stream 30 withdrawn from
the hydrogen pipeline 2. The compressor 32 is conventionally
controlled to maintain the inlet pressure at a target suction
pressure to maintain energy efficient operation of the compressor
32. Although not illustrated, hydrogen compressor 32 can
conventionally incorporate a series of stages with interstage
cooling between stages and an aftercooler to remove the heat of
compression.
The compressed hydrogen stream 34 is introduced into the salt
cavern 3 to form the stored hydrogen 4. The compressed hydrogen
stream 34 flows through a conduit 36 connected to a transfer well
head valve 37 and thereafter into an annular flow area (not shown)
within final well casing 12 (between the inside of final well
casing 12 and brine string 10) from which the compressed hydrogen
feed stream 34 enters salt cavern 3. Well head valve 37 typically
remains in an open position. In the illustrated embodiment, the
compressed hydrogen stream 34 entering the salt cavern 3 is
preferably compressed to a pressure above the pipeline pressure
within the hydrogen pipeline 2 and as a result, the stored hydrogen
4 has a cavern pressure that is above pipeline pressure. Although
not shown, the quantity of the compressed hydrogen stream 34
compressed by compressor 32 can be determined by instrumentation
located at the inlet of the compressor 32. For example, the flow
can be measured by an orifice meter, which is then corrected by the
measured pressure and temperature. Such instrumentation may also
provide a basis for assessing the integrity of the compressor 32,
such as, for example, the detection of leaks. Additionally, the
cavern pressure may be monitored and measured with a pressure
transducer located within the conduit 36 through which the
compressed feed stream 34 flows into the salt cavern 3. A flow
meter may also be positioned at the inlet to the salt cavern 3 to
determine the amount of hydrogen injected into the salt cavern
3.
When a hydrogen generation facility is taken off-line for any
reason or when demand for hydrogen by customers otherwise exceeds
the available production capabilities, stored hydrogen 4 can be
removed from the salt cavern 3 to assist in meeting customer
demand. Outlet flow network 5 is provided for such purposes and
includes the following legs: "A" for flow of a crude hydrogen
stream 38; "B" for flow of a hydrogen dilution stream 40; and "C"
for flow of a hydrogen product stream 42. As used herein and in the
claims, the term "legs" means flow paths within the outlet flow
network 5 that are formed by suitable conduits. Such conduits would
be positioned to conduct the flow of the aforementioned streams
within the outlet flow network 5 as illustrated. Outlet flow
network 5 is put to use by setting valve 35 in a closed position
and a crude hydrogen stream 38 formed from the stored hydrogen 4
flows through a first leg "A" of the outlet flow network 5 by
setting a control valve 41 contained in such leg in an open
position. In a manner that will be discussed, control valve 41 is
controlled so that crude hydrogen stream 38 has a first flow rate
that will assist in meeting customer demand.
Crude hydrogen stream 38, for reasons discussed above, will have a
higher level of contaminants than the hydrogen product
specification and is therefore, processed by being diluted with a
hydrogen dilution stream 40 that is introduced into a second leg
"B" of the outlet flow network 5. As illustrated, second leg "B" is
connected to first leg "A" by means of a piping tee or the like. A
second flow rate of the hydrogen dilution stream 40 is controlled
by a flow control valve 44 located in second leg "B". Depending
upon the pressure of crude hydrogen stream 38, hydrogen dilution
stream 40 can be formed from compressed hydrogen stream 34 by
operating compressor 32 at such time or alternatively, without the
compressor 32, by opening by-pass valve 56 with the compressor 32
shut down. When cavern pressure is low, pipeline pressure within
the hydrogen pipeline 2 can be sufficient to mix the hydrogen
dilution stream 40 with crude hydrogen stream 38. It should be
noted that it is possible to conduct an embodiment of the present
invention in which the cavern pressure would not be sufficient to
withdraw the crude hydrogen stream 38 into mixing vessel 46 for
mixing with the hydrogen dilution stream 40. In such an embodiment,
an additional compressor (not shown) may be used to compress the
crude hydrogen stream 38 that is withdrawn from the salt cavern 3
to a pressure above pipeline pressure.
Within the outlet flow network 5, a mixing vessel 46 is connected
to the first and second legs "A" and "B" such that the hydrogen
dilution stream 40 mixes with the crude hydrogen stream 38 within
the mixing vessel 46, and a product hydrogen stream 42 is produced
that is introduced into a third leg "C" that is connected to the
hydrogen pipeline 2. Any suitable device as known in the art for
mixing gases may be employed. Mixing vessel 46 can be a gas blender
having baffles to increase turbulence of the gases to enhance
mixing. Alternatively, mixing vessel 46 may be a piping manifold
with or without selectively placed mixing elements which creates a
tortuous gas flow pathway to enhance mixing in which the hydrogen
dilution stream 40 and the crude hydrogen stream 38 can flow
co-currently or counter currently relative to each other. It should
be noted that the necessary mixing in the present invention may
also be carried out without the use of a mixing vessel 46.
It is important to point out that hydrogen generation facilities,
such as steam methane reformers, will generally form pipeline
hydrogen having contaminant levels significantly less than the
product purity specification. Consequently, hydrogen dilution
stream 40, composed from the pipeline hydrogen, will tend to have
contaminant levels that are sufficiently lower than the crude
hydrogen stream 38 to ensure that the crude hydrogen stream 38 can
be diluted to at or below the product purity specification. In this
regard, the hydrogen dilution stream 40 will contain contaminants
at levels that are no greater than about 80 percent of the
specification. Contaminant levels within the hydrogen flowing
within hydrogen pipeline 2 that are about 50 percent of the
contaminant levels in the hydrogen product purity specification are
typical. As can be appreciated, higher contaminant levels in the
hydrogen dilution stream 40 will result in a decrease in the
ability to use stored hydrogen 4 because the ability to dilute the
stored hydrogen 4 to form a hydrogen product stream 42 will
decrease.
As mentioned above, compressor 32 may be used to form a hydrogen
dilution stream 40. The amount of compression will at least
partially be dependent upon the pressure of crude hydrogen stream
38 to ensure that the hydrogen dilution stream 40 is sufficiently
pressurized to mix with the crude hydrogen stream 38. In this
regard, a control valve would be opened (not shown) to allow a
portion of the hydrogen from the hydrogen pipeline 2 to be directed
to the inlet of the compressor 32. Bypass valve 56 would be closed
so that hydrogen stream 30 is fed to the inlet of the compressor
32. The compressor 32 includes an aftercooler, which can be
employed to remove the heat of compression. The aftercooler is a
known device in the art consisting of a heat exchanger utilizing
cooling water and a draft fan. The removal of heat from the
compressed hydrogen dilution stream 40 can be performed to achieve
a temperature that is substantially the same as the crude hydrogen
stream 38 to minimize errors between expected and measured flow
rates. For purposes that will be discussed, the second flow rate of
the hydrogen dilution stream 40 is measured by a flow transducer
76. Flow transducer 76, in a manner known in the art, may be
optionally compensated for temperature and pressure effects through
temperature and pressure measurements made by pressure and
temperature transducers (not shown).
Alternatively, if the hydrogen pipeline pressure is sufficiently
high, as compared with cavern pressure within salt cavern 3, the
hydrogen dilution stream 40 may be formed from the hydrogen
pipeline 2 by free flowing the hydrogen directly from the hydrogen
pipeline 2 to mixing vessel 46 without a compressor 32. As also
mentioned above, by-pass valve 56 is set in an open position for
such purposes. Compression of the product stream 42 can be
compressed, if necessary, to ensure that the product stream 42 is
sufficiently pressurized to be introduced into the hydrogen
pipeline 2.
A further option, when compressor 32 is not used in forming the
hydrogen dilution stream 40, is to throttle crude hydrogen stream
38 so that the crude hydrogen stream 38 incurs a pressure drop to
ensure the mixing of crude hydrogen stream 38 with hydrogen
dilution stream 40 in mixing vessel 46. On this point, in most
cases, the expansion of hydrogen dilution stream 40, upon entry
into mixing vessel 46, will result in an increase in temperature by
virtue of its negative Joule Thompson coefficient. As a result,
heat may be removed from the hydrogen dilution stream 40 to ensure
the accurate metering and control of its flow rate prior to mixing
with crude hydrogen stream 38. Again, flow rate measurements of the
hydrogen dilution stream 40 by flow transducer 76 can be
compensated for such temperature effects through temperature
measurements made by temperature transducers (not shown). After
mixing has occurred in mixing vessel 46, the product stream 42 can
be compressed, if necessary, to allow the introduction of the
product stream 42 back into the hydrogen pipeline 2.
It is to be further noted, embodiments of the present invention are
contemplated in which one or more contaminants may be removed from
the crude hydrogen gas stream 38 prior to mixing and/or after
mixing with hydrogen dilution stream 40. For instance, water vapor
may be removed from crude hydrogen stream 38 by a condensation unit
before mixing with hydrogen dilution stream 40 in the mixing vessel
46. However, in the illustrated embodiment, the necessary amount of
contaminated hydrogen is advantageously withdrawn from the salt
cavern 3 as crude hydrogen stream 38 to meet production demand
without the need to remove any contaminants contained therein.
Preferably, the control of the flow rates of the crude hydrogen
stream 38 and the hydrogen dilution stream 40 are automated by
means of a control system 13. The control system 13 can take many
forms that are known in the art. No particular form of such control
for purposes of practicing the present invention is preferred. For
example a supervisory control system can incorporate supervisory
control and data acquisition software that can be obtained from a
variety of sources. Such software can reside on a dedicated
computer that interfaces over a local area network with an
automated control system, such as, for example, a control system
incorporating model predictive control or other automated control
of less sophistication. Connected to the supervisory control system
are controllers 62 and 74 for which targets are set by the
supervisory control system. The controllers 62 and 74 could be
proportional, integral or differential controllers, each of which
is well known in the art and that can be obtained from a variety of
suppliers. Additionally, all of the electrical connections
mentioned above with respect to such control or other controllers
mentioned herein can either be hard wired or radio signals. The
controllers 62 and 74 in turn control flow control valves 41 and
44, respectively and therefore, the first and second flow
rates.
Analyzer banks 81 and 82 are also provided. Impurity concentration
measurements are performed by analyzer banks 81 and 82. Control
system 13, in a manner to be discussed, sets the flow rate targets
for the controllers 62 and 74 on the basis of impurity
concentration measurements within crude hydrogen stream 38 and
hydrogen stream 30. Analyzer bank 81 measures the hydrogen purity
level and concentration of impurities within the hydrogen stream
30. Analyzer bank 82 measures the concentration of hydrogen and the
various contaminants (e.g., H.sub.2, THC, H.sub.2S, H.sub.2O,
CO2/CO, N.sub.2 and O.sub.2) of the crude hydrogen stream 38.
Although not illustrated, suitable microprocessor based designs of
the analyzers 81 and 82 may correct for temperature and pressure,
thereby improving the sensing accuracy of the sensing elements of
the analyzers 81 and 82. Analyzers 81 and 82 may comprise any
suitable type of sensing element as known in the art. For instance,
electrochemical sensor elements may be employed.
A sample line 83 is provided to feed a sample of the hydrogen
stream 30 to each sensing element (e.g., H.sub.2, THC, H.sub.2S,
H.sub.2O, CO2/CO, N.sub.2 and O.sub.2) that is contained in
analyzer bank 81. Each of these impurities diffuse into the sensing
element and each of the sensors undergo an electrochemical reaction
in which current is generated. The amount of electrical current
generated is proportional to the amount of contaminants present in
each of the sensors. A signal that is referable to the measured
impurity concentration is then transmitted through electrical
connection 86 from the analyzer bank 81 to the control system 13.
Similarly, analyzer bank 82 is configured to measure hydrogen and
impurity contamination contained in crude hydrogen stream 38.
Sample line 84 feeds a sample of the crude hydrogen stream 38 to
each sensing element that is contained in analyzer bank 82. A
signal that is referable to the measured impurity concentration is
then transmitted through electrical connection 85 from the analyzer
bank 82 to the control system 13. The sampling of crude hydrogen
stream 38 is intended to be a representative measurement of the
composition of the hydrogen and the contaminants of the stored
hydrogen 4. Alternatively, an analyzer probe (not shown) or other
suitable instrumentation as known in the art may be directly
inserted into the well casing 12 for measurement of the hydrogen
purity level and the other contaminants of the stored hydrogen 4
within the salt cavern 3. It should be also understood that a
single analyzer can measure both the hydrogen stream 30 and the
crude hydrogen stream 38.
A sample line 88 is also provided for the measurement of each of
the impurities THC, H.sub.2S, H.sub.2O, CO2/CO, N.sub.2 and O.sub.2
contained in hydrogen product stream 42 by the analyzer bank 82. A
signal referable to the hydrogen and signals corresponding to the
contaminants levels are transmitted through electrical connection
85 from the analyzer 82 to the control system 13.
The stored hydrogen 4 is withdrawn from the salt cavern 3 as a
crude hydrogen stream 38. In this regard, the flow rate of the
crude hydrogen stream 38 is regulated to a predetermined set point
value by a control valve 41. The controller 62 controls the degree
of opening of the control valve 41 through electrical connection 60
such that the flow rate of crude hydrogen stream 38 is regulated to
a target flow rate Fc that is set by a supervisory level of control
provided by a control system 13. This target flow rate is the flow
rate required to be added to the hydrogen pipeline 2 in order to
assist in meeting the increased demand. A flow transducer 70,
connected to the controller 62 by an electrical connection 66,
provides a signal to the controller 62 that is referable to the
flow rate of the crude hydrogen stream 38 and the controller 62
regulates the degree of opening of control valve 41 so that the
flow of crude hydrogen stream 38 meets the target flow rate. The
controller 62 and control system 13 are connected to one another by
electrical connection 64 so that the controller 62 can receive the
target flow rate signal from the control system 13. A controller 74
responsive to a target flow rate for the hydrogen dilution stream
40 that is computed by control system 13 operates the control valve
44 through electrical connection 77. A flow transducer 76 generates
a signal referable to the flow rate of the hydrogen dilution stream
40 that is fed to the controller 74 as an input through electrical
connection 78 and the controller 74 controls operation of control
valve 44 so that the flow rate of the hydrogen dilution stream 40
is at the target flow rate level. The controller 74 is connected to
the control system 13 through electrical connection 72 to receive a
signal referable to the target flow rate of hydrogen dilution
stream 40. The target flow rate that is generated by control system
13 is that flow rate that will ensure necessary dilution of the one
or more contaminants of the crude hydrogen stream 38 to produce a
hydrogen product stream 42 having a product purity specification
that can be reintroduced into the hydrogen pipeline 2.
With reference to FIG. 2, a control logic diagram 200 is
illustrated that is incorporated into control system 13. In this
embodiment, the deficiency in hydrogen demand is inputted and read
into the control system 13 at step 210. This hydrogen demand
deficiency sets the flow rate (Fc) of the crude hydrogen stream 38
to be withdrawn from the salt cavern 3. It should be noted that the
present invention may also be operated to assist meeting customer
demand so that the crude hydrogen stream 38 is withdrawn at a
predetermined flow rate from the salt cavern 3 on a predictive
basis, which may or may not satisfy the deficiency in hydrogen
demand.
The target hydrogen composition and contaminant limits for the
hydrogen product stream 42 are inputted and read by the control
system 13. As mentioned above, hydrogen and impurity concentrations
from the hydrogen pipeline 2 and the crude hydrogen stream 38 are
sampled by analyzers 81 and 82, respectively, and thereafter
transmitted as input values to the control system 13.
Next, integrity checks, at step 220, are performed to ensure that
the current readings by analyzer 81 and 82 are reliable and
accurate before using them in subsequent computations. Four
integrity checks are conducted, each of which is required to be
successfully met. The first integrity check requires the control
system 13 computes a standard deviation based on the current
reading and the historical data utilizing a sum of squares method
as generally known in the art. The computed standard deviation is
then compared against a predetermined minimum standard deviation to
determine if it is greater than the predetermined minimum standard
deviation to confirm whether the analyzers 81 and 82 are properly
functioning. The predetermined minimum standard deviation is an
inputted value based upon historical data. It is typically
dependent upon a variety of factors, including for example the
specific analyzer and the concentration variation typically
observed for each of the measured contaminants in the hydrogen
storage and processing facility 1. A computed standard deviation
less than the predetermined minimum may be an indicator that the
sensors of the analyzers 81 and 82 are not detecting the presence
of hydrogen as well as their respective contaminants. The control
system 13 also performs two integrity checks to confirm that such
measurements are above a lower analyzer integrity limit and an
upper analyzer integrity limit to ensure that the measured values
are within a predetermined range that is typical for that measured
reading. The lower and higher integrity limits are arbitrary values
typically inputted by a user familiar with the behavior of the
process and analyzer. The fourth integrity check requires the
control system 13 to compute whether there is a percentage increase
from the current reading to the previous reading and, if so, to
determine whether such increase exceeds a predetermined value.
Similar to the foregoing user inputs, this predetermined value is
an arbitrary inputted value based upon on a variety of factors,
including the specific analyzer employed and the concentration
variation typically observed for hydrogen and each of the measured
contaminants in the hydrogen storage and processing facility 1. If
each of the four integrity checks passes, then the control system
13 determines that the analyzers are functioning properly and
therefore the data is reliable for subsequent use in computations
to be performed by the control system 13.
If one or both of the analyzers 81 and 82 are determined to not
pass each of the four integrity checks, then the last stored values
recorded in the control system 13 for that respective analyzer 81
or 82 can be utilized while notifying a technician to address the
situation, as shown in step 225 of FIG. 2. The control logic 200
may limit the number of times the integrity checks will be allowed
to fail before aborting the automated control system for mixing and
allowing for manual intervention for the required mixing. These
safeguards can ensure that the analyzers 81 and 82 are functioning
properly and have not experienced any significant transient spikes
or system perturbations. Assuming the integrity checks at step 220
for the concentration data measured by the analyzers 81 and 82 have
been validated, the measured values are deemed reliable. The
control logic 200 continues at step 230 to calculate the required
flow rate (Fp) of hydrogen dilution stream 40 to mix with and
dilute contaminants in the crude hydrogen stream 38 having a target
flow rate of Fc. First, the control system 13 determines a Fp
required to dilute each of the contaminants contained in the stored
hydrogen 4. Different contaminant levels may require varying levels
of Fp required for dilution to the product purity specification. As
a consequence, for each contaminant having an impurity level that
exceeds specification, the control system 13 performs overall
material balance and contaminant material balances to calculate the
flow rate (Fp) required for diluting the contaminant to a product
purity specification. The control system 13 repeats the computation
for each contaminant having an impurity level exceeding the product
purity specification.
Next, the control system 13 determines which of the contaminants
require the most dilution. Accordingly, the required flow rate (Fp)
of hydrogen from the hydrogen pipeline 2 will be governed or
dominated by the dilution required for such contaminant. In this
manner, the control logic 200 calculates and determines the flow
rate (Fp) to be withdrawn from the hydrogen pipeline 2 such that
the crude hydrogen stream 38, when withdrawn from the stored
hydrogen 4 in the salt cavern 3, will have a contaminant level that
is at or below the product purity specification, whereby each of
the contaminants is diluted to at least the product purity
specification.
When the control logic 200 has calculated the required Fp to be
withdrawn from the hydrogen pipeline 2, the computed levels for
hydrogen and each of the contaminants (X.sub.E,i) of hydrogen
product stream 42 are determined. At this stage, the mixing process
can begin. Control system 13 transmits target flow rates Fp and Fc
to controllers 74 and 62, respectively, as shown at step 240. In
particular, controller 74 receives a signal referable to the target
flow rate Fp of the hydrogen dilution stream 40 through electrical
connection 72, and controller 62 receives a signal referable to the
target flow rate Fc of the crude hydrogen stream 38 through
electrical connection 64.
The hydrogen dilution stream 40 and the crude hydrogen stream 38
are metered and controlled by their respective control valves 44
and 41. The hydrogen dilution stream 40 and the crude hydrogen
stream 38 enter mixing vessel 46 to mix for a predetermined time
period. Thereafter, the mixed stream emerges as hydrogen product
stream 42. Corresponding sensors from analyzer bank 82 obtain a
physical measurement of hydrogen product stream 42 at sample line
88. Signals are then transmitted from the analyzer bank 82 to the
control system 13 that are referable to the hydrogen level and the
impurity levels in hydrogen product stream 42. Integrity checks for
each of the measured readings of hydrogen product stream 42 are
performed as described above. If each of the integrity checks
passes, the control system 13 then generates a model error for the
measured hydrogen and each measured contaminant, as shown in step
245. A model error is evaluated by the control system 13 at step
250. The model error represents the relative error between the
measured and calculated concentration for a particular component in
the hydrogen product stream 42 and is computed as the deviation
between a reconciled value and the expected or computed composition
(as determined by the material balances discussed above). The
reconciled value is a product of the current measured reading and
the trailing or historical bias (i.e., current measured
reading*trailing bias). By way of an example, if the water vapor
analyzer from the analyzer 82 bank is historically reading on
average 10% over the expected value, and the current measurement
read by the water vapor analyzer 82 is 1 ppmv, then the reconciled
value is taken to be 90% of the measured reading (i.e., 0.9 ppmv)
to account for the bias in the water vapor analyzer 82. The bias
can be based upon an instantaneous model error or a moving average
of previous model errors, which are stored by the control system
13.
The calculated model errors for hydrogen and each contaminant are
stored by the control system 13 and represent the updated bias
which can be used for computing the model error of each component
in the hydrogen product stream 42 in the next computing cycle. The
frequency at which the control system 13 repeats the computing
cycle shown in FIG. 2 may be programmed to occur at any time
interval, and is, at least partially based on the rate of variation
in flow rates and compositions of the hydrogen stream 30 and the
crude hydrogen stream 38. In one example, the control system 13
repeats the computation about every two minutes.
It should be noted that to ensure reliability of the measurements
taken at hydrogen product stream 42, a moving average of
measurements may be obtained. In one example, the moving average
consists of measurements repeatedly taken over five minutes.
If the model error for hydrogen and each contaminant are within an
acceptable limit, then the hydrogen product stream 42 is exported
to the hydrogen pipeline 2, as shown at step 260. Acceptable limits
for each of the model errors are dependent upon the particular
component being measured as well as the bias of the analyzer. For
instance, an acceptable limit for one or more contaminants would be
10% of the relative error, where the relative error is defined as
(reconciled value-calculated value)/(calculated value)*100%.
If the model error is outside an acceptable limit, the control
system 13 may send an alarm to the operator. A control valve may
also be provided to automatically stop the flow of hydrogen product
stream 42 from being reintroduced back into the hydrogen pipeline
2. Manual operator intervention may be necessary to increase the
additional amount of hydrogen dilution stream 40 required to
increase the purity of the hydrogen product stream 42 to at least
the product purity specification. Such manual intervention may be
necessary when the control system 13 is programmed with constraints
that limit the maximum amount of incremental hydrogen dilution
stream 40 permitted to be made by control system 13 without manual
intervention. In this case, the control means could be a human
interface to allow a user to note pressure, flow rate and
temperature readings and manually transmit set point values to
controllers 62 and 74. If one or more of the error conditions is
not satisfied at step 250, then the control system 13 and
corresponding controllers 62 and 74 may be aborted to resolve the
reason for the large model error. Even though the hydrogen product
stream 42 is not within product purity specification as determined
by the model errors, it may still be manually exported to the
hydrogen pipeline 2. A decision to manually export the hydrogen
product stream 42 can be appropriate when the volume of the
hydrogen product stream 42 is relatively small in comparison to the
volume of the hydrogen pipeline 2 (e.g., a ratio of 1:8 of the
hydrogen product stream 42 to the pipeline stream on a volume
basis) or the hydrogen product stream 42 marginally deviates from
the product purity specification. In either case, the hydrogen
product stream 42 can be adequately diluted within the hydrogen
pipeline 2 to the product purity specification. Alternatively, the
customer may be willing to accept the hydrogen product stream 42
when the customer is notified of the potential purity variance.
It should be noted that the control system 13 utilized to control
the flow rate of the hydrogen dilution stream 40 (Fp) may operate
based upon various other types of control approaches. For example,
a pure feedback control methodology may be employed in which an
initial estimate for the flow rate of the hydrogen dilution stream
40 is utilized based on the flow rate of crude hydrogen stream 38
withdrawn from the salt cavern 3. The initial estimate may be based
upon on a variety of factors, such as for example, overall material
balance, previously stored values of Fp or upon a simulated model
utilized to predict Fp that is a function of a particular flow rate
of the crude hydrogen stream 38. The hydrogen product stream 42 may
be measured and thereafter, the flow rate of the hydrogen dilution
stream 40 can be incrementally ramped up or ramped down
proportionally, on a manual basis, until the desired composition of
hydrogen product stream 42 is attained. The crude hydrogen stream
38 withdrawn from the salt cavern 3 can then be further
incrementally ramped up and the flow rate of the hydrogen dilution
stream 40 correspondingly increased, both of which can occur in a
step-wise fashion until the desired flow rate Fc is achieved.
Because of the slow rise at which concentration of the various
contaminants may be observed to change in this particular hydrogen
storage and processing facility 1, proper adjustments and ample
response time to such adjustments are possible, thereby rendering
such a feedback control methodology a viable approach. In this
manner, the flow rate Fp of the hydrogen dilution stream 40 is
regulated and fine tuned in response to the incremental ramp up of
the flow rate of the crude hydrogen stream 38.
The control system 13 may also operate based upon pure feed forward
control when the process behavior is well understood, the
composition of the contaminants in the crude hydrogen stream 38
change slowly and there are no significant unexpected process
disturbances. A pure feed forward control approach would predict
the required flow rate Fp of the hydrogen dilution stream 40 and
then regulate the flow rate to such predicted flow rate Fp value
without measuring the composition of the hydrogen product stream
42. The concentration of the contaminants in the crude hydrogen
stream 38 and the hydrogen dilution stream 40 can be measured as a
basis for predicting the required flow rate Fp of the hydrogen
dilution stream 40. The required hydrogen dilution stream 40 can
then be based upon predictive simulation calculations.
Alternatively, the concentration of contaminants in the crude
hydrogen stream 38 may not be measured when the impurity levels
will not appreciably change from the previously measured values.
Such a scenario is possible when the salt cavern 3 is not
substantially depleted of stored hydrogen 4 and/or the flow rate of
the crude hydrogen stream 38 is relatively low. In such a case,
historical values for the concentrations for the crude hydrogen
stream 38 may be utilized to predict the required flow rate of the
hydrogen dilution stream 40.
Further, although an automated control system has been discussed,
it should be understood that the present invention can be
implemented manually. For instance, a user may manually adjust the
flow rates of crude hydrogen stream 38 and the hydrogen dilution
stream 40 by virtue of forecasting the hydrogen demand in the
hydrogen pipeline 2.
While it has been shown and described what is considered to be
certain embodiments of the invention, it will, of course, be
understood that various modifications and changes in form or detail
can readily be made without departing from the spirit and scope of
the invention. It is, therefore, intended that this invention not
be limited to the exact form and detail herein shown and described,
nor to anything less than the whole of the invention herein
disclosed and hereinafter claimed.
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